[PantsOnFire]

Open means the first action by a player who is not in the blinds. So the two choices are open-limp or open-raise (and now that I read the below posts, I remembered open-fold). Technically, SB opened the betting and BB min-raised in blind games. But "open" usually refers to the player that acts first after the BB.

As for your collusion question, I don't think you are allowed to talk about checking it down whether it's a live hand or between hands. You basically cannot discuss team strategy at the poker table.

You are allowed to show displeasure at SB here by saying "what is the purpose of bluffing at a dry side pot?". But you have to be careful not to mention checking it down.

As a side note, there are times that checking it down makes sense and there are other times where a player might wish to let the bubble continue. If I am a big stack and there are two medium stacks and a small stack, I might just want small stack to stay around. Perhaps the medium stacks are simply folding until the bubble bursts. In that case, I will want to build my big stack even bigger because once the bubble bursts, the medium stacks are going to get more aggressive.

In general though, it is a bonehead move to bluff into a dry side pot when somebody is on the brink of elimination.